1. Field of the Invention
Broadly, the present invention relates to the treatment of a hydrocarbon material to produce more valuable products. More particularly, the invention relates to the treatment of a hydrocarbon material in the presence of a molten salt to convert a substantial fraction of the feedstock into more valuable products by way of thermal cracking, catalytic cracking, or hydrocracking. Still further, the invention relates to a process for carrying out the hydrocarbon treatment wherein the hydrocarbon and products are carried in an entrained flow with molten salt.
2. Description of Prior Art
In the treatment of hydrocarbons such as coal, coal-derived products, and petroleum products or byproducts such as residua, progress has depended to a large extent upon improvements in processes which are designed to convert partially refined hydrocarbon fractions into more valuable products. The conversion processes rely, almost without exception, upon the application of heat. Two basic types of processes are utilized: those using heat and hydrogen, and those using only heat.
Among those not using hydrogen, thermal cracking or pyrolysis is perhaps the oldest and most highly developed process. Thermal cracking involves splitting a large hydrocarbon molecule into smaller molecules at elevated temperatures. A portion of these smaller molecules represents olefins and stable light hydrocarbons, such as gasoline, refinery gas, light oils, and gas oils. The remaining products are generally referred to as heavy oils and coke. The process is generally referred to as thermal cracking when the feed is a gas oil or topped crude oil. If, on the other hand, the feed consists of a residual material, it is generally treated by thermal processes known as viscosity breaking or coking. Viscosity breaking is a mild reduction in molecule size and is usually conducted on a once-through basis, whereas the latter is a more severe cracking operation in which a significant portion of the feed is converted to solid petroleum coke. Conventional thermal cracking processes generally result in the formation of some coke and other low-grade products which can result in plugging of system components.
Hydrocracking is the decomposition of hydrocarbons at elevated temperatures and usually high pressures in the presence of hydrogen. Hydrocracking also may be accomplished in the presence of a catalyst. The objective of hydrocracking is to simultaneously hydrogenate the products while cracking the feedstock. The presence of hydrogen also has been found to suppress the formation of tar and coke to some extent.
A number of problems are involved in all of these processes, including catalyst deterioration caused by sulfur, ammonia, or ash which frequently are present in the feedsrock, coke buildup in the reactor components, and deactivation of the catalyst by coke deposition, or mineral matter which is frequently present in a petroleum residual feedstock. Thus, it is seen that each of the above-noted conversion processes is still in need of improvement.
Various molten salts have been suggested as reaction media for treating hydrocarbon materials. For example, U.S. Pat. No. 3,252,773 suggests the use of an alkali metal carbonate or hydroxide melt to gasify coal. U.S. Pat. No. 3,252,774 suggests the use of the molten salts for the production of hydrogen gases from hydrocarbon materials. U.S. Pat. No. 3,862,025 suggests the use of molten oxides, hydroxides, or mixtures thereof for the cracking of heavy hydrocarbon feedstocks. U.S. Pat. Nos. 3,745,109 and 3,871,992 are exemplary of patents which suggest the use of molten alkali metal carbonates for cracking hydrocarbon materials. In the COSMOS process, developed by Mitsui, crude oil is cracked in an externally heated tubular furnace to form olefins (Yamaguchi, F. et al., "COSMOS Cracks Crude to Olefins," Hydrocarbon Processing, September 1979, pp 163-172). It is disclosed that a thin film of molten salt on the walls of the furnace can suppress coke formation and plugging of the reactor tube. They also disclose, however, that if the metal sulfide concentration exceeds about 0.28%, even the special alloys they developed will not withstand the corrosive effect at elevated temperatures. U.S. Pat. No. 3,647,358 suggests the use of a variety of alkali metal halides among other salts for use in the treatment of hydrocarbon materials.
The principal problem with the use of molten salts is that many of them are highly corrosive. Further, those which are not inherently corrosive may become so during use. More particularly, during the processing of a sulfur-containing hydrocarbon feedstock, the sulfur reacts with, for example, sodium carbonate to form sodium sulfide. Even small amounts of sodium sulfide in a carbonate greatly increase its corrosivity. Thus, at the elevated temperatures required for thermal cracking of the hydrocarbon, the presence of even a small amount of sulfide in the sodium carbonate melt requires that all surfaces coming into contact with the melt be protected by expensive ceramic materials which greatly increase processing costs.
Clearly, there is need for an improved process which could minimize the production of low-value products such as petroleum coke and pitch, substantially reduce the problem associated with coke depositionn on equipment surfaces, and also substantially eliminate or reduce the corrosion problems associated with utilizing a molten salt.